A bipartite interaction with the processivity clamp potentiates Pol IV-mediated TLS

Processivity clamps mediate polymerase switching for translesion synthesis (TLS). All three E. coli TLS polymerases interact with the β₂ processivity clamp through a conserved clamp-binding motif (CBM), which is indispensable for TLS. Notably, Pol IV also makes a unique secondary contact with the clamp through non-CBM residues. However, the role of this "rim contact" in Pol IV-mediated TLS remains poorly understood. Here we show that the rim contact is critical for TLS past strong replication blocks. In in vitro reconstituted Pol IV-mediated TLS, ablating the rim contact compromises TLS past 3-methyl dA, a strong block, while barely affecting TLS past N²-furfuryl dG, a weak block. Similar observations are also made in E. coli cells bearing a single copy of these lesions in the genome. Within lesion-stalled replication forks, the rim interaction and ssDNA binding protein cooperatively poise Pol IV to better compete with Pol III for binding to a cleft through its CBM. We propose that this bipartite clamp interaction enables Pol IV to rapidly resolve lesion-stalled replication through TLS at the fork, which reduces damage induced mutagenesis.